Sorry, I did not mean the render quality. In the cloud shader, under the MAIN tab, there is an input for density shader, Edge sharpness is the first slider. For Altocumulus clouds, it defaults to 1. I would set this to 3 or 5. It should not increase the render time.
What the slider does is help give the clouds more definition. So for really high clouds, 1 works great. If my cloud layer was below 3000 meters, I increase this a little. um, It helps make the noise of the density shader clearer.